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I've got a newer road bike I use for my longer, faster weekend rides (700c tires) and a 70's Sekai that I use to ride with my son, and to run errands (27" tires).
I have no problem changing tires/tubes on the 700c set, but I always have a terrible time changing the 27" set. I usually end up pinch-flatting the tube while trying to wrestle the tire/tube onto the wheel.
Am I doing something wrong? Does anyone else have this problem?
Without looking at your wheel, I will have to guess that it isn't the diameter of the wheel that is causing the problem but rather the rims. Some rims come with deeper channels than others. The 2 example cross sections bellow will show you the difference I am referring to. The rim on the left has a deep channel where the tire bead can sit as you remove the tire from the opposite side of the rim. The rim on the right has a shallow channel where squeezing the tire to the middle does little to help you as you remove the tire.
The purpose for the differences is that the more shallow channel gives you a stronger rim at the expense of making it harder to remove the tire. The deeper channel allows you easier tire removal but compromises the strength of the rim. My front rim is like the one on the right and it is a TOTAL pain in the butt to change tires!!
What I use are the Parks Tools Tire Lever, it takes a lot of abuse but is really gentle on the tires/tubes. It will still pinch if you are not careful. Deflate the tire COMPLETELY, slide everything to one side and lift only one bead of the tire over the lip and hook the lever to a spoke. Keep doing this inch by inch using a new lever each time (there are 3) until that one side of the tire comes off, then pull out only the tube. Getting it back on is about the same, but I use dish soap to lube the tire and rims, helps a lot!
I often find a huge deal of variation due to the tire itself. I've had the same size tires from different brands behave very differently. Sometimes they will become easier to remove and re-install after they are worn in a bit.
Sometimes I have to use tire levers to lift the last bit of tire over the lip of the rim, this isn't usually recommended, but sometimes neccesary for extra stubborn tires. If you do resort to this method, then be careful not to pinch the intertube with the levers and be sure that it's not pinched between the rim and the tire when you are done.
